The present exemplary embodiments relate to novel phosphor compositions. They find particular application in conjunction with converting LED-generated ultraviolet (UV), violet or blue radiation into green light or other colored light for use in green traffic signals. It should be appreciated, however, that the invention is also applicable to the conversion of LED and other light source radiation for the production of green light for other applications, such as display lights, etc.
Light emitting diodes (LEDs) are semiconductor light emitters often used as a replacement for other light sources, such as incandescent lamps. They are particularly useful as display lights, warning lights and indicating lights or in other applications where colored light is desired. The color of light produced by an LED is dependent on the type of semiconductor material used in its manufacture.
Colored semiconductor light emitting devices, including light emitting diodes and lasers (both are generally referred to herein as LEDs), have been produced from Group III-V alloys such as gallium nitride (GaN). To form the LEDs, layers of the alloys are typically deposited epitaxially on a substrate, such as silicon carbide or sapphire, and may be doped with a variety of n and p type dopants to improve properties, such as light emission efficiency. With reference to the GaN-based LEDs, light is generally emitted in the UV and/or blue range of the electromagnetic spectrum.
By interposing a phosphor excited by the radiation generated by the LED, light of a different wavelength, e.g., in the visible range of the spectrum, may be generated. Colored LEDs are used in a number of commercial applications such as toys, indicator lights, automotive, display, safety/emergency, directed area lighting and other devices. Manufacturers are continuously looking for new colored phosphors for use in such LEDs to produce custom colors and higher luminosity.
One important application of semiconductor LEDs is as a light source in a traffic light. Presently, a plurality of blue-green emitting LEDs containing II-V semiconductor layers, such as GaN, etc., are used as the green light of a traffic signal (i.e. traffic lights).
Industry regulations often require traffic light colors to have very specific CIE color coordinates. For example, according to the Institute of Transportation Engineers (ITE), a green traffic light in the United States is typically required to have emission CIE color coordinates located within an area of a quadrilateral on a CIE chromaticity diagram, whose corners have the following color coordinates:
a) x=0.000 and y=0.506;
b) x=0.224 and y=0.389;
c) x=0.280 and y=0.450; and
d) x=0.000 and y=0.730.
The following CIE color coordinates are most preferred for green traffic light applications: x=0.1 and y=0.55.
Likewise, industry regulations require automotive display colors to have specific CIE color coordinates. According to the Society of Automotive Engineers (SAE), a green automotive display, such as a vehicle dashboard display, is typically required to have emission CIE color coordinates located within an area of a quadrilateral on a CIE chromaticity diagram, whose corners have the following color coordinates:
e) x=0.0137 and y=0.4831;
f) x=0.2094 and y=0.3953;
g) x=0.2879 and y=0.5196; and
h) x=0.0108 and y=0.7220.
The color coordinates (also known as the chromaticity coordinates) and the CIE chromaticity diagram are explained in detail in several text books, such as on pages 98-107 of K. H. Butler, “Fluorescent Lamp Phosphors” (The Pennsylvania State University Press 1980) and on pages 109-110 of G. Blasse et al., “Luminescent Materials” (Springer-Verlag 1994), both incorporated herein by reference.
Presently, GaN based LEDs are designed to emit blue-green light with a peak wavelength of 505 nm, which has the desired CIE color coordinates of x=0.1 and y=0.55. Such devices include LEDs having an In1−x GaxN active layer manufactured according to desired parameters. However, LEDs with the In1−xGaxN active layer suffer from the following disadvantage. Due to frequent deviations from desired parameters (i.e., manufacturing systematic variations), the LED peak emission wavelength typically deviates from 505 nm, and thus, its CIE color coordinates deviate from the desired x=0.1 and y=0.55 values. For example, the LED color output (e.g., spectral power distribution and peak emission wavelength) varies with the band gap width of the LED active layer. One source of deviation from the desired color coordinates is the variation in the In to Ga ratio during the deposition of the In1−xGaxN active layer, which results in an active layer whose band gap width deviates from the desired value. This ratio is difficult to control precisely during mass production of the LEDs, which leads to inconsistent color coordinates in a given batch of LEDs. Thus, the In1−xGaxN LEDs which are suitable for use in traffic lights have a lower production yield because a large number of such LEDs with unsuitable emission color coordinates have to be discarded.
In addition, green LEDs used in green traffic signals based upon InGaN technology are generally inefficient (˜30 lm for Lumileds Luxeon green LEDs), especially when compared to blue/violet (380-480 nm) based LED light sources. Higher efficiency green devices will lead to brighter traffic signals and/or more inexpensive signals since the number of LEDs required will be reduced. The present invention is directed to overcoming or at least reducing the problems set forth above through the use of new phosphor blends.